a. Field of the Invention
This invention relates to an irradiation apparatus with an electron beam, and more particularly to an apparatus to uniformly irradiate wire shaped, string shaped or tape shaped objects with an electron beam so as to increase the utilization rate of the electron beam.
B. Description of the Prior Art
Recently, an irradiation technique utilizing an electron accelerator has vastly been used in various fields such as the preservation of foods, the sterilization of pharmaceuticals, the hardening of coatings, the polymerization of composite materials such as plastics and woods, cross-linking for the purpose of improving the properties of plastics, and the like.
However, it is difficult to irradiate an object having a circular cross-section at the center of which is disposed a copper or aluminium conductor, such as an insulated cable with uniform density of an electron beam and with a high utilization rate of the beam. That is, an electron beam has a low penetration rate compared with gamma rays, X rays or the like. For example, the maximum penetration distance of electrons having the energy of 2 million electron volts (2 MeV) is about 1 cm in water, and their ionization ability becomes maximum at a position which is only slightly spaced from the irradiation surface of the water. The ionization rate of the electrons may be decreased as the depth from the water surface is increased. If the penetration depth indicating the ionization rate of the water surface is used to define an effective electron penetration distance, the effective penetration distance of an electron at 2 MeV is about 0.65 cm in water. The effective penetration distance is 0.073 cm in the case of copper with a specific gravity of 8.9. Therefore, when the insulated cable is irradiated with the electron beam, the greater part of the electron beam may be dissipated at the conductor of the cable, so that the insulating coating layer at the opposite surface of the conductor can not be irradiated with the electron beam. Techniques for eliminating the described disadvantages of the electron beam irradiation are known in U.S. Pat. No. 2,858,442, U.S. Pat. No. 2,887,583, U.S. Pat. No. 2,897,365, French Pat. No. 1,149,703 or the like. In the irradiation technique disclosed in U.S. Pat. No. 2,858,442, a wire shaped object to be irradiated is engaged between two rollers, and a plurality of pairs of rollers are used to try to increase the uniformity of the electron beam irradiation and the utilization rate of the electron beam. Nevertheless, the uniformity of electron beam irradiation has not been actually increased as much as expected. It is supposed that this occurs from the fact that the electron beam runs downward only and the side of the wire opposite to the irradiated side is shadowed by the conductor of the insulated wire to be irradiated. Further, in the irradiation techniques disclosed in U.S. Pat. No. 2,887,583 and U.S. Pat. No. 2,897,365, the path of the electron beam is deflected, using a magnet so as to uniformly irradiate an object therewith. In this case, however, such techniques may have disadvantages in that a complicated configuration of the magnetic poles is required and in that the path of the electron beam in air becomes long. Another technique for irradiating a cable having a comparatively large diameter with the electron beam is disclosed in French Pat. No. 1,149,703.